well actually it's like this:
visible: students can see it and click on it.
hidden: students cannot see it or access it. If you provide a link to a hidden resource, students will see an error message telling them that the resource is not available to them. Trainers see the resource greyed out to show it's hidden to students.
stealth: resource is not visible to students but trainers can provide a link to the resource and when students click on the link the resource will be displayed.
I hope this helps,